Silver nanowire shrinks path to foldable devices

4 April 2017

EDN Asia

A team of researchers has developed a wire that can be fashioned into a mesh that conducts current and bends easily.

Many of today's touchscreens are made of ceramic material that has only two of three needed qualities—it's conductive and transparent, but it's not flexible. At the University of Vermont, however, scientists have discovered that they could make wires that are not only super strong but are also "stretchy like gum."

The team, led by mechanical engineering professor Frederic Sansoz, worked with silver nanowires just a few hundred atoms thick, and the result was a kind of wire that could be fashioned into a mesh that conducts current, allows light to shine through and even bends easily "you might be able to tie your smartphone into a knot."

Sansoz, his collaborator Scott Mao at the University of Pittsburgh, and their colleagues have led pioneering research on how to transform soft metals, including gold, into super-strong wires at the nanoscale. It’s part of a growing area of research that shows that as materials are engineered to be smaller and smaller it’s possible to eliminate many defects at the atomic scale. “And this makes them much stronger,” Sansoz said, “generally, smaller is stronger.”

But there’s a problem. “As you make them stronger, they become brittle. It’s chewing gum versus window glass,” Sansoz said.

As wires of silver are made smaller and smaller, down to about 40nm, they follow the expected trend: they get relatively stronger and more brittle. But earlier research by other scientists had shown that at even-more-extreme smallness—below 10nm—silver does something weird. “It behaves like a Jello gelatin dessert,” Sansoz said. “It becomes very soft when compressed, has very little strength and slowly returns to its original shape.”

Materials scientists believe this happens because the crystals of silver are so small that most of their atoms are at the surface, with very few interior atoms. This allows diffusion of individual atoms from the surface to dominate the behaviour of the metal instead of the cracking and slipping of organised lattices of atoms within. This causes these tiniest, but solid, silver crystals to have liquid-like behaviour even at room temperature.

“So our question was: what’s happening in the gap between 10 nanometres and 40 nanometres?” said Sansoz. “This is the first study to look at this range of diameters of nanowires.”

What the team of scientists found in the gap—using both an electron microscope and atomistic models on a supercomputer—is that “the two mechanisms coexist at the same time,” Sansoz said. This gives silver wires in that little-explored zone both the strength of the “smaller-is-stronger” principle with the liquid-like weirdness of their smaller cousins. At this Goldilocks-like size, when defects form at the surface of the wire as it’s pulled apart, “then diffusion comes in and heals the defect,” Sansoz said. “So it just stretches and stretches and stretches—elongating up to 200%.”

There has been remarkable progress since 2010 in applying silver nanowires in electronics, Sansoz said, including conductive electrodes for touchscreen displays. And some companies are working hard to apply these wires to creating cost-effective flexible screens. “But, right now, they’re manufacturing totally in the dark,” Sansoz said. “They don’t know what size wire is best.” His new discovery should give chemists and industrial engineers a target size for creating silver wires that could lead to the first foldable phones.